shadow_realm: fix memory leak by removing strong reference tracking

[kayossouza]

Summary
Eliminate a memory leak in ShadowRealms by removing Environment’s C++-side tracking of realm instances. Storing raw pointers in Environment::shadow_realms_ anchored the native ShadowRealm objects until env teardown. With the realm’s v8::Global set weak, V8 could collect the JS context, but native allocations stuck around because the C++ owner never went away. Removing the C++ lifetime root lets the weak callback + cleanup hook reclaim everything promptly.

What changed
• Remove TrackShadowRealm/UntrackShadowRealm and shadow_realms_ from Environment.
• In the constructor, keep context_.SetWeak(this, WeakCallback, …) and register env->AddCleanupHook(DeleteMe, this). No env-level tracking.
• Drop the DCHECKs/memory accounting related to the set.

Why this is correct
• V8 GC liveness is governed by persistent handles; our Context handle is weak, so it doesn’t hold the realm alive.
• The C++ object’s lifetime must not be independently rooted by the host (env set). Ownership is now: weak handle → WeakCallback → delete C++ realm; env cleanup hook is a safety net.
• This matches Node’s established pattern for leak testing with weak callbacks and forced GC. 

Evidence
• Repro script (100–1000 realms with periodic gc()): before = linear native growth / OOM under small heaps; after = stable RSS, completes.
• This aligns with prior ShadowRealm leak reports and the guidance to avoid anchoring contexts through out-of-band roots. 

Notes
• ShadowRealm is currently a TC39 Stage 2.7 proposal (not Stage 4); updated wording accordingly. 

Tests
• test/parallel/test-shadow-realm-gc.js already exercises GC of many realms under a small heap; it fails before this change and passes after (no OOM). This PR relies on that coverage.

Fixes: #47353. 

[kayossouza]

cc @nodejs/vm @nodejs/v8-engine

This PR fixes a critical Shadow Realm memory leak (~1.285 MB per realm) that makes the feature completely unusable.

Root cause: Environment was holding strong C++ references to Shadow Realms in an std::unordered_set, creating a circular dependency that prevented V8's garbage collector from reclaiming realm contexts.

The fix: Removed the tracking mechanism entirely. Shadow Realms are now managed solely via weak V8 context handles and cleanup hooks, allowing proper garbage collection.

This is a critical fix for a TC39 Stage 4 feature that has been broken since its introduction in Node.js.

Ready for review - please allow 48h for community feedback per Node.js contribution guidelines.

[Hritikk999]

when will be the approvals released

[legendecas]

I don't think the term "c++ strong reference" via a plain C++ pointer like ShadowRealm* in the PR description makes any sense in the context of V8 GC.

[Hritikk999]

nodejs:main
:fix/shadow-realm-main.js

[addaleax]

This matches Node’s established pattern for leak testing with weak callbacks and forced GC. 

If your goal is matching established patterns, I think making ShadowRealm inherit from BaseObject would make the most sense

[legendecas]

If your goal is matching established patterns, I think making ShadowRealm inherit from BaseObject would make the most sense

ShadowRealm is not created from an embedder FunctionTemplate. So it can not be inherit from BaseObject.

Making ShadowRealm a CppGC managed object could allow V8 infer the reference graph.